In the past year I've noticed on some big sites a newer ad format, basically wide skyscraper type ads which remain in view in a sidebar area as the viewer scrolls down the page.

I'd like to do something similar. Would it be possible for an ad which begins out of view to remain in view once the visitor goes to scroll past it?

I would be wanting to setup this type of ad as the last item in my sidebar area. This way for longer pages the sidebar has an ad that remains in view rather than blank unused whitespace.

Any ideas on where I might find a script that works like this?

Thanks in advance for any help.